Harmonic Generation Research Articles

Dion-Jacobson Perovskites with a Ferroelectrically Switchable Chiral Nonlinear Optical Response.

Electrically switchable second harmonic generation (SHG) is highly valuable in electro-optic modulators, which can be deployed in data communication and quantum optics. Coupling circular dichroism (CD) with an electrically controlled SHG process is advantageous because it enhances the signal transmission bandwidth and security while enabling multiple modulation modes for optical logic. However, ferroelectrically switchable chiral second-order nonlinearity is rarely reported. Here, we demonstrate the electro-optic modulation of SHG based on the multipolar effect in a chiral ferroelectric two-dimensional hybrid perovskite s-(FBDA)CdCl4 (s-FBDA = (2s)-2-fluorobutane-1,4-diamine). s-(FBDA)CdCl4 (s-FCC) crystals are highly stable in air, and its SHG response exhibits circular dichroism (i.e., SHG-CD) that can be ferroelectrically switched. The ferroelectrically switchable SHG-CD arises from the reconfiguration of distinct chirality-induced multipolar moments during the ferroelectric switching process. This reconfiguration induces a π/2 circular difference in SHG, leading to a reversal of circularly polarized light sensitivity. As a proof of concept, we demonstrate that the electrically controlled SHG-CD of s-FCC can be used in the transmission of encrypted optical communications.

Cationic Coordination Modification Drives Birefringence and Nonlinear Effect Double Lifting in Sulfate.

As nonlinear optical (NLO) crystals, sulfates have the superiority of transparency for ultraviolet (UV) light, but they are often troubled by small nonlinear coefficients and birefringence owing to the high symmetry of the [SO4]2- group. By introducing two neutral diethylenetriamine (DETA) molecules to replace the six coordinated water molecules of the [Zn(H2O)6]2+ complex cation in [Zn(H2O)6](SO4)(H2O), a new sulfate with an acentric structure, namely, [Zn(DETA)2](SO4)(H2O)3, has been designed and synthesized. Structural investigation reveals that the coordination modification of Zn2+ ion tremendously enhances its intraoctahedral distortion. The formed distorted [Zn(DETA)2]2+ cations and the [SO4]2- groups feature an optimized arrangement, endowing [Zn(DETA)2](SO4)(H2O)3 with enhancements in both second harmonic generation (SHG) intensity, from undetectable to 0.25 × KH2PO4 (KDP), and birefringence, from 0.014 to 0.042 at 1064 nm. Despite the slight compromise made in the light transmission range, [Zn(DETA)2](SO4)(H2O)3 still possesses a short absorption edge of 220 nm, retaining the majority of the light transmission range in the solar-blind region. Our work provides a novel and applicable approach of cationic coordination modification to improve the nonlinear optical coefficient and birefringence of sulfates.

Even harmonic generation in semiconductors below and above the band gap assisted by an intense terahertz field

Even harmonic generation in semiconductors below and above the band gap assisted by an intense terahertz field

Three-dimensional bonding anisotropy of bulk hexagonal metal titanium demonstrated by high harmonic generation

High harmonic generation (HHG) in solid-state materials is an emerging field of photonics research that can unveil the detailed electronic structure of materials, bond strengths and scattering processes of electrons. Although HHG in semiconducting and insulating materials has been intensively investigated both experimentally and theoretically, metals have rarely been explored because the strong screening effect of high-density free electrons is considered to significantly weaken the HHG signal. Here, we investigated HHG upon infrared excitation in bulk hexagonal metal titanium (Ti), a typical building block for practical lightweight structural materials. By analyzing the polarization dependence, the approach revealed the three-dimensional (3D) anisotropy in the electronic states. The results demonstrated the potential of HHG spectroscopy for characterizing 3D bonding anisotropy in metallic systems that are of fundamental importance for designing lightweight and strong structural materials.

Li3[AlP2O7F(OH)](H2O)0.5 and A[Al2(PO4)2F(H2O)](A = K, Rb): Counter Cation Templates Derived Three Polar Alkaline Metal Fluoroaluminophosphates.

Three novel alkali metal fluoroaluminophosphates, Li3[AlP2O7F(OH)](H2O)0.5 and A[Al2(PO4)2F(H2O)](A = K, Rb), were designed and synthesized by using low-temperature flux methods. They crystallized in polar space groups P4 and P212121, respectively. Li3[AlP2O7F(OH)](H2O)0.5 features a unique two-dimensional layered structure of fluoroaluminophosphate [AlP2O7F(OH)]3-n, composed of alternately connected AlFO5 octahedra and PO4 tetrahedra. In contrast, with counter cations from Li+ to K+/Rb+, two new crystals of A[Al2(PO4)2F(H2O)](A = K, Rb) have been obtained. They possess a distinct three-dimensional anionic framework, [Al2(PO4)2F]-n, consisting of corner-sharing AlO5 triangular bipyramids, AlFO5 octahedra, and PO4 tetrahedra, revealing intersecting open tunnels along the [010], [001], and [111] directions. Notably, both Li3[AlP2O7F(OH)](H2O)0.5 and K[Al2(PO4)2F(H2O)] exhibit moderate second harmonic generation effects. Besides, porous material of K[Al2(PO4)2F(H2O)] exhibited remarkable ionic exchange properties with Cs+, implying its potential utility in the remediation of radioactive waste and offering a promising solution for managing nuclear contaminants. This research reports on their syntheses, topological structures, elemental analysis, thermal stability, IR Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy, ion exchange property, and nonlinear optical characteristics.

The Study on the Propagation of a Driving Laser Through Gas Target Using a Neural Network: Interaction of Intense Laser with Atoms

High-order harmonic generation is one of the ways to generate attosecond ultra-short pulses. In order to accurately simulate the high-order harmonic emission, it is necessary to perform fast and accurate calculations on the interaction between the atoms and strong laser fields. The accurate profile of the laser field is obtained from the propagation through the gas target. Under the conditions of longer wavelength driving lasers and higher gas densities, the calculation of the laser field becomes more challenging. In this paper, we utilize the driving laser electric field information obtained from numerically solving the three-dimensional Maxwell’s equations as data for machine learning, enabling the prediction of the propagation process of intense laser fields using an artificial neural network. It is found that the simulation based on frequency domain can improve the accuracy of electric field by two orders of magnitude compared with the simulation directly from time domain. On this basis, the feasibility of the transfer learning scheme for laser field prediction is further studied. This study lays a foundation for the rapid and accurate simulation of the interaction between intense laser and matter by using an artificial neural network scheme.

Neural-network enabled octave-spanning coherent diffraction imaging

Ultrafast lasers, providing the shortest pulses worldwide, have been playing a vital role in the ultrafast imaging technology. The temporal resolution has been increasing rapidly in recent years but finally reaches its limit—the pulse width approaches photoperiods, causing significant broadening of spectral bandwidth. The state-of-the-art high harmonics generation based attosecond lasers, with pulse widths reaching ∼50 attoseconds, present octave-spanning spectra. This brings a major challenge to traditional imaging methods, as they result in unbearable chromatic aberrations. To address this challenge, we propose the neural-network approach for broadband imaging and demonstrate its effectiveness empirically by facilitating rapid coherent diffractive imaging under octave-spanning supercontinuum illumination. The proposed method remains effective when deployed with three-octave-spanning spectra, supporting both continuous and comb-like profiles as indicated by simulations. Such lensless imaging method, applicable to both extreme ultraviolet and soft x-ray sources, potentially provides an approach to attosecond imaging.

Two-Photon and Three-Photon Circular Dichroism of Au38 Gold Nanoclusters Enantiomers.

While circular dichroism (CD) and optical activity (OA) are well established as optical effects used in characterization of chiral media, harmonic generation and multiphoton circular dichroism are increasingly seen as new, convenient ways of exploring chirality thanks to their operation in the near-infrared range of wavelengths. However, quantitative data about two-photon circular dichroism (2PCD) of organic and inorganic materials are scarce, and even less can be found about three-photon circular dichroism (3PCD). Here, we show that both 2PCD and 3PCD can be readily detected in chiral atomically precise gold nanoclusters via polarimetric Z-scan technique. We provide quantitative data on 2PCD and 3PCD of both enantiomers of Au38(PET)24 nanoclusters, which exhibit extraordinary chiroptical properties arising from the interplay of several levels of molecular organization. Interestingly, the corresponding two- and three-photon dissymmetry factors of Au38(PET)24 enantiomers are significantly enhanced in comparison to the one-photon CD by factors of 178 and 217, respectively.

Tunable FSRS measurements with reduced background signals: Using an etalon filter to generate picosecond pump pulses in the 460-650nm range.

Generating wavelength-tunable picosecond laser pulses from an ultrafast laser source is essential for femtosecond stimulated Raman scattering (FSRS) measurements. Etalon filters produce narrowband (picosecond) pulses with an asymmetric temporal profile that is ideal for stimulated resonance Raman excitation. However, direct spectral filtering of femtosecond laser pulses is typically limited to the laser's fundamental and harmonic frequencies due to very low transmission of broad bandwidth pulses through an etalon. Here, we show that a single etalon filter (15cm-1 bandwidth; 172cm-1 free spectral range) provides an efficient and tunable option for generating Raman pump pulses over a wide range of wavelengths when used in combination with an optical parametric amplifier and a second harmonic generation (SHG) crystal that has an appropriate phase-matching bandwidth for partial spectral compression before the etalon. Tuning the SHG wavelength to match individual transmission lines of the etalon filter gives asymmetric picosecond pump pulses over a range of 460-650nm. Importantly, the SHG crystal length determines the temporal rise time of the filtered pulse, which is an important property for reducing background and increasing Raman signals compared with symmetric pulses having the same total energy. We examine the wavelength-dependent trade-off between spectral narrowing via SHG and the asymmetric pulse shape after transmission through the etalon. This approach provides a relatively simple and efficient method to generate tunable pump pulses with the optimum temporal profile for resonance-enhanced FSRS measurements across the visible region of the spectrum.

Role of short and long trajectories on the quantum-optical state after high-order harmonic generation

Role of short and long trajectories on the quantum-optical state after high-order harmonic generation

MIR laser CEP estimation using machine learning concepts in bulk high harmonic generation

Monitoring the carrier-envelope phase (CEP) is of paramount importance for experiments involving few-cycle intense laser fields. Common measurement techniques include f-2f interferometry or stereo-ATI setups. Here we demonstrate a new concept, both by simulations and by experiments, for CEP estimation in the mid-infrared regime using machine learning (ML) techniques that rely on the observation of the spectrum of high harmonic generation (HHG) in bulk material. Once the ML model is trained, the method provides a way for cheap and compact in-situ CEP tagging. This technique can complement other CEP monitoring methods, can capture the complex correlation between the CEP and the observable HHG spectra, and is readily generalizable for any laser wavelengths.

An Effective Method for Generating Isolated Attosecond Pulses from a Solid Crystal Film

Solids subjected to strong-field laser excitation can produce high harmonics, making high-order harmonic generation (HHG) one of the most effective methods for creating ultrafast coherent light sources, such as isolated attosecond pulses (IAPs). While extensive research has been conducted on generating IAPs through HHG in gaseous media, studies focusing on solid media are relatively limited. In crystals, the presence of numerous ionization and recombination sites, combined with high density and periodic structure, results in more complex interference dynamics. This complexity paves the way for unique applications in generating IAPs. Using an argon (Ar) crystal as a specific example, we have proposed and theoretically demonstrated an innovative approach for generating IAPs from a solid crystal film using a multi-cycle conventional driving laser pulse.

Ultrafast optical induction of magnetic order at a quantum critical point

Time-resolved ultrafast spectroscopy has emerged as a promising tool to dynamically induce and manipulate non-trivial electronic states of matter out-of-equilibrium. Here we theoretically investigate light pulse driven dynamics in a Kondo lattice system close to quantum criticality. Based on a time-dependent auxiliary fermion mean-field calculation we show that light can dehybridize the local Kondo screening and induce oscillating magnetic order out of a previously paramagnetic state. Depending on the laser pulse field amplitude and frequency the Kondo singlet can be completely deconfined, inducing a dynamic Lifshitz transition that changes the Fermi surface topology. These phenomena can be identified in harmonic generation and time-resolved angle-resolved photoemission spectroscopy spectra. Our results shed new light on non-equilibrium states in heavy fermion systems.

Curvature-assisted high harmonic generation in strongly-driven superconductors

The dynamic of the order parameter in superconductors (SCs) under strong driving is inherently nonlinear, but utilization for nonlinear optics and high harmonic generation (HHG) is hindered by the weak coupling of SCs to transverse homogeneous driving fields. When the superconducting coherence length is large enough to allow for curvature modulations without breaking the superconducting phase, the coupling of the order parameter to the driving-field vector potential changes. Here, we show that with the help of full numerical simulations, mesoscopic curvatures or bending of strongly driven type-II SC structures result in highly nonlinear THz superconducting currents, which lead to far-field HHG extending up to the tens harmonics. It is shown that even and odd harmonics can be controllably emitted by a transport current. The driving mechanism is a nonlinear steering of supercurrent by geometric and finite-size effects. At the same time, the SC state remains fully coherent over the whole sample and within the driving time. The phase matching of the emitted harmonics is discussed.

Comparative analysis of biopolymer films derived from corn and potato starch with insights into morphological, structural and thermal properties

Starch biopolymer films were prepared using the solvent casting method involving acetic acid hydrolysis and glycerol plasticization. This process facilitated a more uniform distribution of plasticizers within the starch matrix, enhancing the films' flexibility. Fourier-transform infrared (FTIR) and Raman spectroscopy confirmed the formation of ester linkages and structural changes in the biopolymer films, attributed to glycerol integration. The optimal formulation comprised 6% starch, 6.8% acetic acid, and 6.8% glycerol. X-ray diffraction (XRD) analysis revealed a reduction in crystallinity of the starch during film formation, enhancing flexibility. Second harmonic generation (SHG) and coherent anti-Stokes Raman scattering (CARS) microscopy indicated that potato starch films had higher crystallinity compared to corn starch films. Thermal analysis via differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) showed that potato starch films exhibited lower gelatinization temperatures and higher thermal stability compared to corn starch films. Functional characterization demonstrated that higher starch content decreased water solubility and water vapor transmission rate, while increasing starch content improved the film's structural integrity. The films were hydrophilic, with static water contact angles indicating moderate wettability. Degradation studies showed that the films were stable in neutral and basic conditions but degraded under acidic conditions over time. The results suggest that potato starch films, with optimized glycerol and acetic acid content, offer improved flexibility, thermal stability, and structural integrity compared to corn starch films. Their performance in various conditions highlights their potential for specific applications, particularly where moisture and environmental stability are critical.

Design of High-Performance Infrared Nonlinear Optical PAs3S3 with Perfectly Aligned Polar Molecular Cage via a Bipolar-Axis-Symmetry Coupling Strategy.

Strong polar molecular cages have recently emerged as novel functional building units for high-performance infrared nonlinear optical (IR NLO) crystals. However, these highly polar molecular cages often arrange themselves in a way that cancels out their polarity, leading to a more energetically stable state. As a result, most cage crystal formations tend to crystallize in centrosymmetric space groups, which conflicts with the primary requirement for NLO crystals. Herein, we address the challenge of polar molecular cage arrangement through bipolar-axis-symmetry coupling strategy, utilizing classical NLO parent compounds. By substituting the C3v symmetric [B3O6] groups with polar C3v symmetric [PAs3S3] cages within the β-BBO polar aixs lattice, we successfully synthesized a new compound, PAs3S3 (PAS), which exhibits a consistent arrangement of polar molecular cages-crucial for maximizing NLO performance. Additionally, due to the non-covalent interactions among [PAs3S3] polar molecular cages, PAS demonstrates an unexpectedly strong second harmonic generation (SHG) about 8 times that of AgGaS2, along with a significant bandgap of 2.75 eV. Furthermore, PAS exhibits remarkable stability against air and moisture. These findings validate our design strategy and position PAS as a promising candidate for applications in IR NLO crystals.

Phase Engineering of Giant Second Harmonic Generation in Bi2O2Se.

2D materials with remarkable second-harmonic generation (SHG) hold promise for future on-chip nonlinear optics. Relevant materials with both giant SHG response and environmental stability are long-sought targets. Here, the enormous SHG from the phase engineering of a high-performance semiconductor, Bi2O2Se (BOS), under uniaxial strain, is demonstrated. SHG signals captured in strained 20 nm-BOS films exceed those of NbOI2 and NbOCl2 of similar thickness by a factor of 10, and are four orders of magnitude higher than monolayer-MoS2, resulting in a significant second-order nonlinear susceptibility on the order of 1 nmV-1. Intriguingly, the strain enables continuous adjustment of the ferroelectric phase transition across room temperature. An exceptionally large tunability of SHG, approximately six orders of magnitude, is achieved through strain modulation. This colossal SHG, originating from the geometric phase of Bloch wave functions and coupled with sensitive strain tunability in this air-stable 2D semiconductor, opens new possibilities for designing chip-scale, switchable nonlinear opticaldevices.

Tunable second harmonic generation in 2D materials: Comparison of different strategies

Nonlinear optical frequency conversion, where optical fields interact with a nonlinear medium to generate new frequencies, is a key phenomenon in modern photonic systems. However, a major challenge with these techniques lies in the difficulty of tuning the nonlinear electrical susceptibilities that drive such effects in a given material. As a result, dynamic control of optical nonlinearities has remained largely confined to research laboratories, limiting its practical use as a spectroscopic tool. In this work, we aim to advance the development of devices with tunable nonlinear responses by exploring two potential mechanisms for electrically manipulating second-order optical nonlinearity in two-dimensional materials. Specifically, we consider two configurations: in the first, the material does not inherently exhibit second-harmonic generation (SHG), but this response is induced by an external field; in the second, an external field induces doping in a material that already exhibits SHG, altering the intensity of the nonlinear signal. In this work, we have studied these two configurations using a real-time ab-initio approach under an out-of-plane external field and including the effects of doping-induced variations in the screened electron-electron interaction. We then discuss the limitations of current computational methods and compare our results with experimental measurements.

Photocontrol of ferroelectricity in multiferroic BiFeO3 via structural modification coupled with photocarrier

Ultrafast control of ferroelectricity and magnetism by light is essential for future development in multiple functioning devices. Here, we demonstrate that the intense and ultrafast photo-modulation of the electric dipole can be realized by photocarrier injection into a multiferroic BiFeO3 thin film using optical pump-probe and second harmonic generation measurements. Results of ultrafast electron diffraction with <100 fs time resolution and theoretical study reveal that the localized photocarrier strongly couples with the lattice structure and becomes the origin for the observed sudden change in the electric dipole. In addition, the subsequent structural dynamics involve a strong oscillation with a frequency of ~3.3 THz despite a poor structural symmetry change. Based on a theoretical calculation, this oscillation can be attributed to an unexpectedly softened new phonon mode generated by mixing essential two phonon modes governing the multiferroic (ferroelectric and antiferromagnetic) nature of BiFeO3 in the ground state due to strong coupling with a localized photocarrier. The comprehensive study shows that injection of the localized photocarrier strongly coupled with the lattice vibration mode can simultaneously realize the ultrafast switching of electric dipoles and magnetic interaction at once, even at room temperature, without modifying the long-range lattice structure.

High harmonic generation spectra from polyacetylene in condensed phase

In this work, we investigate high-harmonic generation (HHG) spectra in polyacetylene with two relevant bands by numerically solving the time-dependent Schrödinger equation. Using the Su-Schrieffer-Heeger (SSH) model, the problem reduces to a 2 × 2 tight-binding Bloch Hamiltonian. Additionally, coupling to the laser field requires solving the time-dependent Schrödinger equation numerically in reciprocal space. The HHG spectra are obtained by performing a Fourier transform in two consecutive steps: first on the lattice momentum and then on time. The k -integration of velocity is carried out to ensure the coherent addition of the momentum contributions of all electrons within the Brillouin zone. The time integration of the acceleration yields the HHG spectra. Our findings reveal the well-known signatures of high harmonics from solids.